Aluminum alloys are light weight and have about one third the specific gravity of steel materials, and are also superior in corrosion resistance. Furthermore, since plastic working of aluminum alloys can be carried out easily at low temperatures, they are metallic materials suitable for a reduction in weight of equipment and energysaving. However, aluminum itself is inherently low in strength and inferior in heat resistance and wear resistance. It is therefore unsuitable for use in fabrication of mechanical parts for which are required a high strength, and heat resistance and wear resistance.
Recently, various alloying methods and heat treatments, for example, have been developed. As a result, high performance aluminum materials have been developed and their application in various fields is now under investigation. For example, in 1911, A. Wilm developed high strength aluminum alloys such as Duraluminum, and these aluminum alloys have been widely used in production of air crafts. Duraluminum has a composition of 4% Cu, 0.5% Mg, 0.5% Mn, 0.3% Si, with the balance being Al, and has a tensile strength of about 40 kg/mm.sup.2 (see Hashiguchi ed., Kinzoku Gaku Handbook (Handbook of Metalography), 1958. In addition, as heat resistant and wear resistant materials, aluminum/silicon-base alloys have been developed. They are called "Silmin".TM., in which wear resistance is increased by adding from 10 to 20% by weight of Si particles to the Al matrix. In this case, however, the primary silicon crystals are readily increased in size as the result of addition of a large amount of Si, and the strength is inevitably decreased.
As heat resistant, wear resistant materials, Al--Fe-base and Al--Si-base alloys, for example, are known. At present, an extensive investigation is being made on their application as engine parts of a vehicle, such as piston and cylinder liner. For these heat resistant, wear resistant alloys, it is also required that the coefficient of thermal expansion is low. An aluminum alloy usually has a co-efficient of thermal expansion of more than 22.times.10.sup.-6 /.degree. C. In production of a piston, for example, it is desirable that the aluminum alloy have a coefficient of thermal expansion of not more than 21.times.10.sup.-6 /.degree. C. For many of the conventional Al--Fe-base and Al--Si-base alloys, the coefficient of thermal expansion is more than 21.times.10.sup.6 /.degree. C. Thus they are not suitable for use in the production of a piston, for example.
As alloys produced by powder metallurgy, aluminum sintered bodies in which finely divided aluminum oxide is dispersed in aluminum have been developed under the name of "SAP". They were developed to increase heat resistance, and their strength is 35 kg/mm.sup.2 and thus they are brittle, i.e., they have a disadvantage in that the impact resistance is low. For this reason, they have not yet been put into practical use.
Production of mechanical parts of aluminum alloys by the powder metallurgical method has now been put into practical use. In addition to a method comprising the usual powder compacting the sintering and sizing, a cold forging method in which after sintering, coining is applied is also included. Aluminum alloy metal parts produced by the above powder metallurgical method, however, are inferior in mechanical properties such as tensile strength, wear resistance, and heat resistant strength to those produced by cutting, forging, and casting of melted materials.
Next, an explanation is made as to an improvement of modulus of elasticity in high strength aluminum alloy.
As high strength aluminum alloy materials, a 7000 aluminum alloy and a 2000 aluminum alloy are well known. In recent years, a 7090 aluminum alloy and a 7091 aluminum alloy having a much higher strength have been developed in U.S.A.
Such high strength aluminum alloys are used mainly in the production of aircraft. For these aluminum alloys for aircraft are required to have high elasticity and high strength. It is desirable that the modulus of elasticity and strength be at least 8,500 kg/mm.sup.2 and at least 60 kg/mm.sup.2, respectively. Aluminum alloys now on the market have a tensile strength of about 60 kg/mm.sup.2, but their modulus of elasticity is less than 8,000 kg/mm.sup.2, which is less than 1/2 of that of the iron-base material. Furthermore, it is said that these aluminum alloys are sacrificed in corrosion resistance. In order to produce an aluminum alloy having a high modulus of elasticity, attempts to combine with carbon or ceramic fibers, or particles, or to add lithium, for example, have been made. No satisfactory aluminum alloy has been developed.
For many of mechanical parts which need high wear resistance, high strength and high heat resistance are required at the same time. Thus the above-described conventional aluminum alloys are not suitable for use in the production of such mechanical parts.
The casting method is not acceptable for producing high Si content aluminum alloy. If high Si contained aluminum alloy comprising not less than 10% of Si, not less than 2% of transition element such as Fe and Ni, and Cu and Mg and balance aluminum is produced by the casting method, the size of precipitation elements of Si and Fe is increased upon solidification, so that the desired characteristics, such as high wear resistance, of the resultant alloy deteriorate, and cracks may occur upon casting. The increased size of precipitated crystals may be regulated. The increase of precipitation size may be regulated to some extent by adding phosphorus. However, reduction of precipitation size by phosphorus addition does not permit production of an aluminum alloy having high mechanical properties, such as mechanical strength.